The present invention relates to a focus detection method and apparatus for use in a camera, a microscope, an optically recording and reproducing apparatus and so on.
Heretofore, as for the method for effecting a focus control, there has been provided mainly two methods, one of which is a focus detection system utilizing an image sharpness and the other of which is a focus detection system utilizing a lateral image shift. The abovementioned two focus detection systems have excellent advantages, respectively, but on the other hand they have also inherent disadvantages. Therefore, it is difficult to perform an accurate focus control by using only one method. In the focus detection system by utilizing the image sharpness, highly accurate focus detection can be effected for an image having abrupt light intensity distribution, and thus when this system is applied to an automatic focusing system of a TTL single-lens reflex camera, it is possible to use various lenses each having a different F-number without causing a light eclipse. However, in case of an extremely de-focused condition, since an evaluation value of this system becomes extremely small and a variation of the evaluation value becomes gentle correspondingly, it is not possible to detect a de-focusing direction precisely and thus a lens driving direction cannot be determined. In case of applying this system to a subject having a small contrast variation such as a part of a human face, it is difficult to effect the focusing control precisely even near an in-focused condition because the evaluation value also becomes small. Moreover, in the focus detection system utilizing a lateral image shift, the focusing condition can be detected for the subject having the small contrast variation, and thus it is possible to detect the lens driving direction even in the extremely de-focused condition. However, if this focus detection system is applied to the automatic control of the TTL single-lens reflex camera, light impinging upon the light receiving element is shaded in the case of a small diaphragm number, and thus the accuracy of the focus control is decreased. Moreover, in this case, since the accuracy of the focus control is decreased even for the subject having the small contrast, it is difficult to effect the focus detection precisely. That is to say, when utilizing the image sharpness focus detection system, the detection sensitivity is low for the image including a low spatial frequency component. Contrary to this, when utilizing the lateral shift focus detection system, the detection sensitivity is low for the low contrast image (wherein a difference in the light intensity between a maximum value and a minimum value is small). Further, the lateral shift focus detection system has such a disadvantage that near the in-focused condition a detection gain becomes small theoretically and thus it is not possible to perform the focusing control accurately.
In order to eliminate the drawbacks mentioned above, in a co-pending U.S. patent application an apparatus for effecting the focus controlling by combining two focus detection systems has been proposed. In the apparatus utilizing these two systems, the focus detection is performed in such a manner that use is made of the lateral shift focus detection system in the de-focused region and near the in-focused condition use is made of the image sharpness focus detection system, by taking into consideration their detection characteristic. However, in the image sharpness focus detection system, there is a condition wherein a detection signal cannot be derived even near the in-focused condition for the essentially low contrast image other than for the de-focused image mentioned above. In this case, even if the focus detection system is changed near the in-focused condition from the lateral shift one to the image sharpness one, it is not possible to perform the focus detection in an accurate manner.